A differential amplifier is a well-known type of amplifier circuit, which provides an output signal that is proportional to the difference between two input signals. An ideal differential amplifier is designed to amplify the difference between the two input signals. Differential amplifiers of the type discussed herein have both differential inputs and differential outputs. Typically, such amplifiers exhibit a fixed closed-loop gain established by feedback-resistors and gain-resistors. The common mode gain of this type of amplifier is low.
FIG. 7 shows a differential amplifier circuit 20 comprising an amplifier 21 having positive (+) and negative (−) inputs. The amplifier 21 amplifies the difference between the signals applied to its positive (+) and negative (−) inputs and provides amplified positive (+) and negative (−) outputs, on lines OUTN and OUTP. The amplifier 21 exhibits an output impedance between the output terminals.
Circuit 20 includes feedback-resistors (Rfn, Rfp) and gain-resistors (Rip, Rin) connected to the inputs of amplifier 21. Positive input PSIG is connected through gain-resistor Rip to the positive input terminal of amplifier 21. Negative input NSIG is connected through gain-resistor Rin to the negative input terminal of amplifier 21. Feedback from the negative output OUTN is applied, through feedback resistor Rfn, to the positive input terminal of amplifier 21. Feedback from the positive output OUTP is applied, through feedback resistor Rfp, to the negative input terminal of amplifier 21. The values of the gain resistors (Ri) and the feedback resistors (Rf) establish the closed-loop gain of the amplifier circuit 20.
“Common mode” in general is the average value of two or more signals. The amplifier circuit 21 of FIG. 7 will produce the difference between the two outputs OUTP and OUTN. However, without further processing, the circuit will not produce the average or common mode between those two outputs. For example, assume there is a 1 volt difference between inputs PSIG and NSIG, and the resistors establish a gain of unity. There will be a 1 volt difference in voltage between the two outputs OUTP and OUTN. However, each output voltage OUTP and OUTN can be at any voltage level (so long as the two outputs are 1 volt apart), because the average or common mode is not established.
The common mode level of the output of a differential amplifier of the type described may be established using feedback. The average value of the two outputs is measured and compared to a desired value. Common mode can be measured at the node interconnecting two equal value resistors (impedances) of a voltage divider. A current or voltage proportional to the difference between the measured common mode and the desired value is generated. This current or voltage is then applied to bias the differential amplifier so as to influence the output common mode. A negative feedback loop acts to cause the output common mode to track closely to the desired value.
FIG. 8 illustrates a prior art implementation for setting the common mode level of the output of a differential amplifier using feedback. In this example, the main amplifier circuit 20 is of a type shown in FIG. 7 and described above. A pair of equal value resistors Ra and Rb are connected across the differential output lines OUTN and OUTP. The voltage at the node between resistors Ra and Rb represents the average value of the two outputs. An operational amplifier 23 compares average value at its input (+) to a desired common mode value Vocm at its input (−). The amplifier 23 produces a signal proportional to the difference between the measured average and the desired common mode value Vocm, which is applied to bias the differential amplifier 21, so as to influence the output common mode. The negative feedback loop through amplifier 23 acts to drive the output common mode to the desired value Vocm.
A disadvantage of the described circuit (FIG. 8) is that the common-mode feedback loop can be difficult to compensate or stabilize. The signal path from the control input of the main amplifier (where the amplifier receives the control/feedback signal) to each amplifier output can be a high frequency signal path and is a part of the common mode feedback loop. Such a path may have several poles/phase shifts, and tends to oscillate. Stabilizing such a circuit may be difficult. Also, there can be interactions between the common-mode feedback loop and the main (differential) feedback loop, which can further complicate the design.